Can indoor fantail goldfish be put outside in a pond?

Yes, but you need to make sure you do not shock the fish when you change the water temperature. To do this properly; you need to put the fish in a plastic bag with water from the aquarium it is in, put the bag with the fish in it on the water of the pond, the fish and bag will float on top. Leave this there for about an hour before opening the bag and pouring the fish gently into the pond. Make sure there is plenty of water in the bag that the fish can swim in if the bag floats sideways to prevent any injury to the fish.